Scientists have long sought efficient ways to convert methane and other hydrocarbons into longer chain hydrocarbons, olefins, aromatic hydrocarbons, and other products. CH bond activation has been the focus of intense research for decades, with mixed results. More efficient processes may create value in a number of ways, including facilitating the utilization of remotely located hydrocarbon feedstocks (such as stranded natural gas) through conversion into more easily transportable and useful fuels and feedstocks, and allowing the use of inexpensive feedstocks (e.g., methane and other light hydrocarbons) for end products often made from higher hydrocarbons.
U.S. Pat. No. 6,525,230 discloses methods of converting alkanes to other compounds using a “zone reactor” comprised of a hollow, unsegregated interior defining first, second, and third zones. Oxygen reacts with metal bromide in the first zone to provide bromine; bromine reacts with the alkane in the second zone to form alkyl bromide and hydrogen bromide; and the alkyl bromide reacts with metal oxide in the third zone to form the corresponding product. In one embodiment, the flow of gases through the reactor may be reversed to convert the metal oxide back to metal bromide and to convert the metal bromide back to the metal oxide. The reactor may essentially operated in a cyclic mode.
Other processes may include an oxidative halogenation process for producing alkyl halides from an alkane, hydrogen halide, and, preferably, oxygen, using a rare earth halide or oxyhalide catalyst. A metal halide catalyst may also be used for oxidative halogenation of alkanes. Oxidative halogenation, however, has several disadvantages, including the production of perhalogenated products and an unacceptable quantity of deep oxidation products (CO and CO2).
Other processes include a bromine-based process for converting gaseous alkanes to liquid hydrocarbons. Several basic steps may be used, including (1) reacting bromine with alkanes to produce alkyl bromides and hydrobromic acid (bromination), (2) reacting the alkyl bromide and hydrobromic acid product with a crystalline alumino-silicate catalyst to form higher molecular weight hydrocarbons and hydrobromic acid (coupling), (3) neutralizing the hydrobromic acid by reaction with an aqueous solution of partially oxidized metal bromide salts (as metal oxides/oxybromides/bromides) to produce a metal bromide salt and water in an aqueous solution, or by reaction of the hydrobromic acid with air over a metal bromide catalyst, and (4) regenerating bromine by reaction of the metal bromide salt with oxygen to yield bromine and an oxidized salt. Potential drawbacks of the processes may include low methane conversions; short space-times and the resulting potential for less than 100% bromine conversion; wasteful overbromination of ethane, propane, and higher alkanes, resulting in the formation of dibromomethane and other polybrominated alkanes, which will likely form coke under the disclosed reaction conditions; comparatively low alkyl bromide conversions; the need to separate the hydrocarbon product stream from an aqueous hydrohalic acid stream; and inadequate capture of halogen during the regeneration of the catalyst to remove halogen-containing coke. In addition, the proposed venting of this bromine-containing stream may be both economically and environmentally unacceptable.
The process described above may also requires operation at relatively low temperatures to prevent significant selectivity to methane. One result may be incomplete conversion of alkyl bromide species and, because the process relies on stream splitting to recover products, a considerable amount of unconverted alkyl bromides may leave the process with the products. This represents an unacceptable loss of bromine (as unconverted methyl bromide) and a reduced carbon efficiency.
The neutralization of hydrobromic acid by reaction with an aqueous solution of partially oxidized metal bromide salts and subsequent reaction of the metal bromide salts formed with oxygen to yield bromine and an oxidized salt may also have a number of disadvantages. First, any carbon dioxide present may form carbonates in the slurry, which may not be regenerable. Second, the maximum temperature may be limited due to pressure increases which are intolerable above approximately 200° C., thus preventing complete recovery of halogen. Third, although the use of redox-active metal oxides (e.g., oxides of V, Cr, Mn, Fe, Co, Ce, and Cu) may contribute to molecular bromine formation during the neutralization of hydrobromic acid, incomplete HBr conversion due to the use of a solid bromide salt may in turn result in a significant loss of bromine from the system (in the water phase). Provided an excess of air was used, the bromide salt might eventually be converted to the oxide form, stopping any further loss of HBr in the water discard.
To separate water from bromine, a process may utilize condensation and phase separation to produce semi-dry liquid bromine and a water/bromine mixture. Other means for separating water from bromine, such as using an inert gas to strip the bromine from the water phase or using adsorption-based methods have also been proposed; however, such methods are minimally effective and result in a significant overall loss of halogen.
An oxychlorination process may first remove the water from HCl (a costly step) and then reacts the HCl with oxygen and hydrocarbon directly. Oxychlorination processes rely on the separation of HCl from the unreacted alkanes and higher hydrocarbon products by using water absorption, and subsequent recovery of anhydrous HCl from the aqueous hydrochloric acid. Processes for the absorption of HCl in water may dissapate the heat of absorption by contacting the HCl gas with ambient air, and also by the vaporization of water. Such processes may produce aqueous hydrochloric acid with a concentration of at least 35.5 wt %. Other processes may allow for the recovery of anhydrous HCl gas by extractive distillation using a chloride salt. Still other processes allow for the production of gaseous HCl from dilute aqueous HCl using an amine together with an inert water-immiscible solvent.
Although researchers have made some progress in the search for more efficient CH bond activation pathways for converting natural gas and other hydrocarbon feedstocks into fuels and other products, there remains a tremendous need for a continuous, economically viable, and more efficient process.